Activated carbon cryogels and related methods

ABSTRACT

Carbon cryogels, methods for making the carbon cryogels, methods for storing a gas using the carbon cryogels, and devices for storing and delivering a gas using the carbon cryogels.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/284,140, filed Nov. 21, 2005, now issued as U.S. Pat. No. 7,723,262,which application is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

The invention was made by an agency of the United States Government orunder a contract with an agency of the United States Government. Thename of the United States Government agency is National ScienceFoundation/Integrative Graduate Education and Research Traineeship, andthe Government contract number is DGE9987620AM006.

FIELD OF THE INVENTION

The present invention relates to carbon cryogels, methods for making thecarbon cryogels, methods for storing a gas using carbon cryogels, anddevices for storing and delivering a gas using carbon cryogels.

BACKGROUND OF THE INVENTION

Recent increases in demand for oil, associated price increases, andenvironmental issues are continuing to exert pressure on an alreadystretched world energy infrastructure. Natural gas, with an estimated60-70 year reserve, represents a clean and abundant fossil fuel thatcould transition from this troubled oil and gasoline dominated market tothe expected eventual adoption of renewable energy and hydrogen.However, one of the hurdles to widespread use of natural gas inautomobiles and power plants is storage of the gas. An ideal gas storagevessel should contain gas at reasonable temperatures and pressures whilemaintaining a low weight, a small volume, and minimal cost. There areproblems associated with highly compressed natural gas (CNG) andcryogenic liquid natural gas (LNG). One of the favored alternatives tothese two storage methods is natural gas adsorbed on a microporousmedium such as activated carbon. Adsorbed natural gas (ANG) hasdemonstrated storage performance competitive with CNG, but at pressuresas low as 3.45 MPa (compared to 15.17 MPa for CNG). This relatively lowpressure allows for easier tank filling, provides room for non-cylinderform factors, allows for optional tank materials and increases thesafety of a tank.

Activated carbon is the dominant material in research on storage ofadsorbed methane and is typically synthesized by pyrolysis (i.e.,carbonization) and activation treatments on existing organic materialssuch as coconut fibers, carbon fibers, and even tire rubber. However,few of these precursor materials can be easily engineered to anysignificant degree.

A need exists for a carbon cryogel having a microporous structure thatcan be tuned by varying sol-gel parameters to produce a carbon cryogelfor low pressure methane storage. The present invention seeks to fulfillthis need and provides further related advantages.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a carbon cryogel having adensity of from about 0.20 to about 1.0 g/cm³, a surface area of fromabout 500 to about 3000 m²/g, a total pore volume of from about 1.0 toabout 1.5 cm³/g, and a gas storage capacity of from about 0.0010 toabout 0.015 mole/g at room temperature and at 500 psi.

In one embodiment, the cryogel has a density from about 0.25 to about0.75 g/cm³.

In one embodiment, the cryogel has a surface area from about 1500 toabout 3000 m²/g.

In one embodiment, the cryogel has a pore volume from about 1.2 to about1.4 cm³/g.

In one embodiment, the cryogel has a gas storage capacity from about0.005 to about 0.010 mole/g.

The carbon cryogel of the invention can store a variety of gases.Representative gases that can be stored by the carbon cryogel includemethane, hydrogen, nitrogen, carbon monoxide, fluorine, nitric oxide,nitrogen trifluoride, silane, ethylene, boron trifluoride, phosphine,arsine, disilane, and carbon tetrafluoride.

The carbon cryogel of the invention is obtainable by the process of:

(a) preparing a sol by mixing resorcinol, formaldehyde, and a catalystin water;

(b) gelling the sol by heating at a temperature and for a timesufficient to provide a gel;

(c) washing the gel with acid to provide an acid-washed gel comprisingan aqueous solvent;

(d) washing the acid-washed gel with a suitable organic solvent toexchange the aqueous solvent for an organic solvent to provide asolvent-exchanged gel;

(e) freeze drying the solvent-exchanged gel; and

(f) pyrolyzing the solvent-exchanged gel to provide a carbon cryogel,

wherein the cryogel has a gas storage capacity of from about 0.0010 toabout 0.015 mole/g.

In another aspect of the invention, a method for making a carbon cryogelis provided. The method includes the steps of

(a) preparing a sol by mixing resorcinol, formaldehyde, and a catalystin water;

(b) gelling the sol by heating at a temperature and for a timesufficient to provide a gel;

(c) washing the gel with acid to provide an acid-washed gel comprisingan aqueous solvent;

(d) washing the acid-washed gel with a suitable organic solvent toexchange the aqueous solvent for an organic solvent to provide asolvent-exchanged gel;

(e) freeze drying the solvent-exchanged gel; and

(f) pyrolyzing the solvent-exchanged gel to provide a carbon cryogel.

In one embodiment, the method further comprising heating the carboncryogel at a temperature and for a time sufficient to provide anactivated carbon cryogel.

In one embodiment, the activation temperature is about 900° C. and thetime is from about 10 to about 120 minutes.

In one embodiment, activation is from about 5 to about 90%.

In one embodiment, the molar ratio of resorcinol to catalyst is fromabout 10 to about 300.

In one embodiment, the catalyst is sodium carbonate.

In one embodiment, the weight ratio of resorcinol to water is from about0.01 to about 2.0.

In one embodiment, gelling the sol comprises heating at a temperatureand for a period of time sufficient to convert the sol to a crosslinkedgel. In one embodiment, gelling the sol comprises heating at about 90°C. for from about 1 to about 7 days.

In one embodiment, washing the gel with acid comprises washing the gelwith aqueous trifluoroacetic acid.

In one embodiment, washing the acid-washed gel comprises washing the gelwith t-butanol.

In one embodiment, pyrolyzing the solvent-exchanged gel comprisesheating at a temperature and for a period of time sufficient to convertthe gel to a carbon cryogel. In one embodiment, pyrolyzing thesolvent-exchanged gel comprises heating at about 1050° C. for about 4hours under nitrogen.

In one embodiment, heating at a temperature and for a time sufficient toprovide an activated carbon cryogel comprises heating at about 900° C.under carbon dioxide.

In another aspect, the invention provides a method for gas storage. Inthe method, gas is stored by contacting a vessel containing an adsorbatewith a gas, wherein the adsorbate comprises a carbon cryogel having adensity of from about 0.20 to about 1.0 g/cm³, a surface area of fromabout 500 to about 3000 m²/g, a total pore volume of from about 1.0 toabout 1.5 cm³/g, and a gas storage capacity of from about 0.0010 toabout 0.015 mole/g at room temperature and at 500 psi. In oneembodiment, the gas is methane.

In another aspect of the invention, a gas storage vessel is provided.The gas storage vessel comprises a pressurizable vessel including acarbon cryogel having a density of from about 0.20 to about 1.0 g/cm³, asurface area of from about 500 to about 3000 m²/g, a total pore volumeof from about 1.0 to about 1.5 cm³/g, and a gas storage capacity of fromabout 0.0010 to about 0.015 mole/g at room temperature and at 500 psi.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1C are images of an RF hydrogel, an RF cryogel, arepresentative carbon cryogel of the invention, respectively, and FIG.1D is a transmission electron microscope (TEM) image (175,000×) of thecarbon cryogel of FIG. 1C;

FIG. 2 is a schematic diagram of an apparatus for measuring methaneadsorption/desorption for the carbon cryogels of the invention;

FIGS. 3A-3D are graphs illustrating nitrogen adsorption isotherms andpore size distributions for representative RF cryogels formed inaccordance with the invention having R/W constant at 0.005 and R/C 50(FIG. 3A (nitrogen adsorption) and FIG. 3B (pore size distribution)) andR/C 300 (FIG. 3C (nitrogen adsorption) and FIG. 3D (pore sizedistribution));

FIG. 4 is a graph comparing surface area (m²/g), pore size (nm), andpore volume (10×cm³/g) of RF cryogel as a function of R/C with R/W molarconstant at 0.005;

FIG. 5 is a graph comparing pore volume (cm³/g) as a function of poresize distribution between 2-4 nm for representative activated (dashedline) and unactivated (solid line) carbon cryogels of the inventionhaving R/C 50;

FIG. 6 is a graph illustrating methane adsorption curves (V/V andmole/g) as a function of pressure at room temperature for arepresentative carbon cryogel of the invention (Sample 7, see Table 1);

FIG. 7A is a graph comparing volumetric (V/V) and gravimetric (mole/g)methane storage performance as a function of R/C for representativecarbon cryogels of the invention (Samples 2, 7, 11, and 18, see Table 1)having R/W 0.25 and activation levels from 67%-78%;

FIG. 7B is a graph comparing surface area and total pore volume as afunction of R/C for representative carbon cryogels of the invention(Samples 2, 7, 11, and 18, see Table 1) having R/W 0.25 and activationlevels from 67%-78%;

FIG. 8A is a graph comparing pore volume as a function of pore diameterfor representative carbon cryogels of the invention (Samples 2 (R/C 10),7 (R/C 25), 11 (R/C 50), and 18 (R/C 75), see Table 1) having R/W 0.25and activation levels from 67%-78%;

FIG. 8B is a graph comparing nitrogen sorption isotherms forrepresentative carbon cryogels of the invention (Samples 2 (R/C 10), 7(R/C 25), 11 (R/C 50), and 18 (R/C 75), see Table 1) having R/W 0.25 andactivation levels from 67%-78%;

FIGS. 9A and 9B compare the volumetric performance (V/V) and density(g/cm³) as a function of R/W, and the gravimetric performance (mole/g)and inverse density (cm³/g) as a function of R/W on loose powder samplesof representative carbon cryogels of the invention (Samples 14, 17, 12,21, and 23, see Table 1) having R/C 75 and activation between 14% and36%; and

FIG. 10 is a graph comparing the effect of activation on methane storageperformance (V/V and mole/g) for representative carbon cryogels of theinvention (Samples 15-19, see Table 1) having R/C 75 and R/W 0.25cryogels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In one aspect, the present invention provides a carbon cryogel. Thecarbon cryogel is a porous sorbent having a surface area and a microporevolume such that gases can be densely adsorbed onto the surface of orcondensed into the cryogel micropores. When gases are adsorbed on thesurface of the cryogel, their density is about that of highly compressedgas. Adsorption using a carbon cryogel of the invention does not requireextreme temperatures or pressures. Therefore, a vessel containing acarbon cryogel of the invention allows the vessel to contain either moregas per unit volume, or a similar amount of gas at lower pressure orhigher temperature.

The carbon cryogel of the invention is a sol gel-derived, highly porous,high surface area material. The cryogel is an interconnectedmicro-/meso-porous carbon material that can adsorb large volumes of gas(e.g., methane) at moderate temperature and pressure. The cryogel canact as a gas storage medium when added to a pressure vessel.

The carbon cryogel is a substantially pure carbon material characterizedas having a large surface area, large pore volume with small pore size,and a relatively high density.

The carbon cryogel has a surface area of from about 500 m²/g to about3000 m²/g. In one embodiment, the cryogel has a surface area of fromabout 1000 m²/g to about 2500 m²/g. In one embodiment, the cryogel has asurface area of from about 1500 to about 3000 m²/g. In one embodiment,the cryogel has a surface area of from about 2000 m²/g to about 3000m²/g. In one embodiment, the cryogel has a surface area of from about2500 m²/g to about 3000 m²/g.

The cryogel has a total pore volume of from about 1.0 to about 1.5cm³/g. In one embodiment, the cryogel has a total pore volume (singlepoint nitrogen adsorption) of from about 1.2 to about 1.4 cm³/g. Thecryogel has a maximum pore size distribution that can be engineered tobe less than about 5 nm. In one embodiment, the cryogel has a maximumpore size distribution of less than about 5 nm. In one embodiment, thecryogel has a maximum pore size distribution of from about 0.5 nm toabout 2.0 nm.

The cryogel has an uncompacted (i.e., loose) powder density of fromabout 0.20 to about 1.0 g/cm³. In one embodiment, the cryogel has adensity of from about 0.25 to about 0.75 g/cm³. In one embodiment, thecryogel has a density of from about 0.30 to about 0.50 g/cm³.

The cryogel has a gas storage capacity of from about 0.0010 to about0.015 mole/g. In one embodiment, the cryogel has a gas storage capacityof from about 0.005 to about 0.010 mole/g.

Representative carbon cryogels of the invention have the following gasstorage capacities (see Table 2):

-   -   mole/g 0.0010-0.0131    -   V/V (loose powder) 22-118    -   % Gravimetric 1.63-21.05

The terms “mole/g” and “V/V” relate to the cryogel's gas (i.e., methane)storage performance. The term “mole/g” relates to the gravimetricstorage capacity and refers to moles methane stored per gram carbon. Thegravimetric storage capacity is indicative of the cryogel's microporousstructure. The term “V/V” relates to the volumetric storage capacity andrefers to the ratio of the volume that the stored gas would occupy atstandard temperature and pressure to the volume that the uncompactedpowder sample occupies. The term “% Gravimetric” relates the moles ofstored gas (i.e., methane) to the mass of stored gas and is defined as100×gas weight/carbon weight.

In another aspect, the present invention provides a gas storage systemthat includes a quantity of a carbon cryogel of the invention and avessel suitable for receiving the carbon cryogel and the gas to bestored.

A carbon cryogel of the invention can be placed in a pressurizeablevessel in either compacted powder form or its natural monolithic form.Due to the relatively low pressures involved, the vessel can be fittedto any number of forms (e.g., non-cylindrical). Electrical contacts canbe placed in a manner such that a current can be passed through thecryogel to enhance gas desorption. Alternately, heating elements can beplaced in the vessel to aid desorption. The vessel can be designed for awide range of storage capacities from relatively small portable units torelatively large stationary tanks. Gas storage capacity depends on thenature of the carbon cryogel used. The storage capacity of a tank isdetermined by multiplying the V/V performance of the particular carboncryogel in use by the internal volume of the storage vessel. For naturalgas storage applications, a filter adsorbent unit can be placed suchthat gas exiting or entering the main storage vessel passes through theadsorbent. The adsorbent in the filter can be a carbon cryogel designedspecially for removing certain contaminants and odorants present in mostnatural gas lines. The filter can be temperature controlled to eitheradsorb or desorb contaminants depending on whether gas is exiting orentering the main storage vessel. Gas enters and exits the system bypressurizing or depressurizing (i.e., to fill the vessel pressure isapplied and to empty the vessel pressure is released. Methane can enterand exit the storage device through pressurized lines including amanifold that allows gas to enter from a methane source (e.g., fillingstation, pump, pipeline) and exit to the desired location (e.g., engine,fuel cell reformer, pipeline).

In another aspect of the invention, methods for making a carbon cryogelare provided. The carbon cryogel is fabricated from resorcinol,formaldehyde, water, and a catalyst. The cryogel can be made by thefollowing representative multi-step sequence:

(1) preparing an initial sol by mixing resorcinol (e.g., 50 parts byweight), formaldehyde (e.g., 100 parts by weight), and sodium carbonate(e.g., 1 part by weight) in water (e.g., to provide a 5% by weightsolution);

(2) gelling the sol by heating at a temperature and for a time (e.g.,90° C. for 1-7 days) sufficient to provide a gel (i.e., hydrogel);

(3) acid washing the gel by agitating in an acid bath to provide anacid-washed gel (e.g., 0.125% by weight trifluoroacetic acid in water at45° C. for 3 days, pH=1.9);

(4) washing the acid-washed gels by solvent exchange (e.g., t-butanol,10 times volume, repeated 3 times) to provide a solvent-exchanged gel;

(5) freeze drying the solvent-exchanged gel (e.g., 3 hours at 263° K andthen vacuum for 3 days) to provide an organic cryogel; and

(6) pyrolyzing the organic cryogel (e.g., 1050° C. for 4 hours undernitrogen) to provide a carbon cryogel.

The carbon cryogel can be optionally activated by heating at elevatedtemperature in a carbon dioxide atmosphere (e.g., 900° C. under carbondioxide) to provide an activated carbon cryogel. Activationsubstantially increases micropore volume and provides carbon cryogelshaving high pore volume, high surface area, and low pore sizes.

In one embodiment, the carbon cryogel is fabricated from a phenoliccompound (e.g., resorcinol (R)), formaldehyde (F), water (W), and acatalyst (C).

Phenolic compounds can be reacted with formaldehyde in the presence of abasic catalyst to provide a polymeric gel (crosslinked gel). Suitablephenolic compounds include a polyhydroxy benzene, such as a dihydroxy ortrihydroxy benzene. Representative polyhydroxy benzenes includeresorcinol (i.e., 1,3-dihydroxy benzene), catechol, hydroquinone, andphloroglucinol. Mixtures of two or more polyhydroxy benzenes can also beused. Phenol (monohydroxy benzene) can also be used.

The ratios of these materials (e.g., R/C and R/W), as well as theprocessing parameters, determine the ultimate structure and propertiesof the product carbon cryogels.

Representative carbon cryogels were prepared from the following rangesof materials and levels of activation with carbon dioxide to increasesurface area (see Table 1):

-   -   R/C 10-300    -   R/W 0.05-2.0    -   % Activation 7-78

R/C is the molar ratio of resorcinol to catalyst used in making thecryogel; R/W is the weight ratio of resorcinol to water used in makingthe cryogel; and % Activation is the percent by weight of originalmaterial removed by the activation process.

For cryogels having R/W=0.25 and 67-78% activation, advantageous BETsurface area (2400 to 2600 m²/g), total pore volume (1.0 to 1.5 cm³/g),and % gravimetric methane (15 to 20) values were observed for cryogelsmade with R/C from about 20 to about 60, particularly 25 and 50.

In one embodiment, the carbon cryogel has a density of from about 0.20to about 1.0 g/cm³, a surface area of from about 1500 to about 2000m²/g, a total pore volume of from about 1.0 to about 1.5 cm³/g, and agas storage capacity of from about 0.0010 to about 0.015 mole/g.

As noted above, the carbon cryogel of the invention is prepared from anorganic hydrogel that is solvent exchanged with a suitable organicsolvent and freeze dried to provide an organic cryogel, which is thenpyrolyzed to provide the carbon cryogel. The carbon cryogel can beactivated by reaction with carbon dioxide at elevated temperature toprovide an activated cryogel. The preparation and characteristics of theorganic cryogels, carbon cryogels, and activated cryogels is describedbelow.

Organic Cryogels. Organic cryogels were prepared by freeze dryingorganic hydrogels. Relatively little volume loss occurred on freezedrying, although freeze drying produced cryogels having some cracks(compare FIGS. 1A and 1B). Nitrogen sorption (i.e., adsorption anddesorption) isotherms and pore size distribution of RF cryogels with R/Cratios of 50 and 300, respectively, are illustrated in FIGS. 3A-3D.Nitrogen sorption isotherms and pore size distributions forrepresentative RF cryogels formed in accordance with the inventionhaving R/W constant at 0.005 and R/C 50 are shown in FIG. 3A (nitrogensorption) and FIG. 3B (pore size distribution), having R/W constant at0.005 and R/C 300 are shown in FIG. 3C (nitrogen sorption) and FIG. 3D(pore size distribution).

As shown in the these figures, a change in R/C from 50 to 300 results ina change in average pore size from about 4 nm to about 16 nm. Pore size,surface area, and pore volume as a function of R/C with R/W molarconstant at 0.05 is shown FIG. 4. The surface area of the RF cryogeldecreases from 780 m²/g to 439 m²/g as R/C increases from 50 to 300. Thepore volume increases from 0.68 cm³/g for R/C 50 to 1.47 cm³/g for R/C300.

The material property changes can be explained by a phase separationthat varies on a scale dependent on the amount of crosslinking thatoccurs as the material gels. If significant amounts of catalyst areaavailable (low R/C), the result is a highly crosslinked polymer networkthat is relatively uniform. The phase separation between the polymer andthe solvent occurs at the nanoscale level and results in a moremicroporous material with high surface area. The overall pore volume isreduced because, although micropores increase the surface area,micropores are relatively small in volume. At the other end of thespectrum, if only small quantities of catalyst are available, then thereaction occurs more slowly with less crosslinking. This allows thematerial to phase separate on a larger scale resulting in more meso- andmacropores and a correspondingly lower surface area. Thus, R/C is a keyvariable in producing the ideal surface area and micropore size.

The observed mechanical strength of RF hydrogels and cryogels variesnoticeably with the sol composition. An increase in R/W results in anincreased hardness of both RF hydrogels and cryogels, while an increasein R/C reduces the hardness of the resultant RF hydrogels and cryogels.Such change in mechanical strength can be ascribed to the strength ofthe gel network. An increased R/W was observed to result in asignificantly reduced gelation time. For example, at a constant R/C of75, the gelation time reduces from 10,000 minutes for R/W of 0.01 to 10minutes for R/W of 1.00. This fast hydrolysis and condensation reactionresults in a dense structured gel network.

Carbon Cryogels. Carbon cryogels were prepared by pyrolyzing organiccryogels. The conversion of RF cryogels to carbon cryogels was typicallyaccompanied by a volume loss (compare FIGS. 1B and 1C). The estimatedvolume loss was typically between about 60 to about 80 percent. Theweight loss during pyrolysis was 47%±4%. The TEM image of arepresentative carbon cryogel is shown in FIG. 1D. Referring to FIG. 1D,the cryogel has a string-of-pearls-like-appearance. Subsequentactivation of the carbon cryogels resulted in an increase in pore volumeparticularly in the microporous range as illustrated in FIG. 5.Activation occurs by the following reaction:C_((s))+CO_(2(g))→2CO_((g))

The representative carbon cryogel depicted in FIG. 5 was activated for10 minutes, so the increase in micropore volume is attributed to arelatively small number of new micropores that were exposed as surfacecarbon material was removed. The activation process can eventually reacha point of diminishing returns where so much material is removed thatthe micropores begin to increase in diameter, thereby reducing theoverall surfaces available for adsorption and diminishing theeffectiveness of pores. It should be noted that FIG. 5 does not presentthe entire pore size distribution and this particular sample alsopossessed a significant mesoporous peak at about 55 nm. The mesoporouspeak was reduced in volume (by about ⅔) after activation. It is presumedthat this is due in part to a sintering effect that is not sufficientlyoffset by the above noted reaction, thus resulting in a net decrease inpore volume.

Methane Storage. Adsorption storage capacity for representative carboncryogels of the invention are tabulated in Table 2. FIG. 6, as anexample, shows the typical methane adsorption curves for arepresentative carbon cryogel of the invention (Sample 7, See Table 1)at room temperature, with V/V (solid line) and gravimetric storage(dashed line) as a function of pressure. V/V is not a material propertyas it is largely dependant on sample compaction. However, gravimetricstorage capacity is indicative of the microporous structure of thecryogel.

The effect of R/C on carbon cryogel methane storage capacity isillustrated in FIGS. 7A and 7B. FIG. 7A compares volumetric (V/V) andgravimetric (mole/g) methane storage performance as a function of R/Cfor representative carbon cryogels of the invention (Samples 2, 7, 11,and 18, see Table 1) having R/W 0.25 and activation levels from 67%-78%.Increasing R/C from 10 to 25 resulted in a dramatic increase in bothgravimetric (about 4 times) and volumetric (about 2 times) storagecapacity. The gravimetric storage capacity decreases substantially fromits maximum of about 13 m mole/g (0.013 mole/g) at R/C 50 as R/C wasincreased to 75. However, the volumetric performance experienced only asmall fluctuation when R/C was increased from 25 to 75. FIG. 7B showsboth pore volume and BET surface area, as determined by nitrogensorption isotherms at 77° K. FIG. 7B compares surface area and totalpore volume as a function of R/C for representative carbon cryogels ofthe invention (Samples 2, 7, 11, and 18, see Table 1) having R/W 0.25and activation levels from 67%-78%. Comparison of FIGS. 7A and 7Bclearly demonstrates a close correlation between the gravimetric storagecapacity and the BET surface area and pore volume.

An R/C value that is either too high or too low drastically decreasesthe storage performance of the material. This behavior is explained interms of the previously mentioned length scale at which phase separationoccurs between the solvent and solid material in the gelling polymernetwork. In the RF system, the length scales of the microphaseseparation of the solid and solvent components are equal. The effect onthe final carbon material is that for low R/C the width of both thepores and the solid portions are very small, whereas high R/C results inan open structure with large pores and correspondingly large solidportions. This rationale explains why a high R/C carbon cryogel exhibitsrelatively poor methane storage capacity. A high R/C carbon cryogelconsists of relatively large solid chords of carbon separated by poresof a size beyond that which is beneficial for gas storage. The reducedperformance of low R/C cryogels may be explained by drying- orpyrolysis-induced collapse of very small pores that result from highestcatalyst concentrations. This agrees with the volume loss observed insome very low R/W cryogels. For a constant R/W of 0.01, R/C 50 cryogelvolume loss was about 70%, whereas R/C 300 cryogel volume loss was onlyabout 50%. Corresponding weight losses for these gels were 47% and 51%for the R/C 50 and R/C 300 cryogels, respectively. Related behavior suchas reduced micropore volume and surface area at high or low R/C was alsoobserved.

FIGS. 8A and 8B show pore size distribution and nitrogen sorptionisotherms, respectively, for the same carbon cryogels noted in FIGS. 7Aand 7B. FIG. 8A compares pore volume as a function of pore diameter forrepresentative carbon cryogels of the invention (Samples 2 (R/C 10), 7(R/C 25), 11 (R/C 50), and 18 (R/C 75), see Table 1) having R/W 0.25 andactivation levels from 67%-78%. Depending on R/C, the carbon cryogelshave either bimodal or trimodal pore size distribution. The mesoporouspeak at 7-8 nm is typical of unactivated carbon cryogels and aerogels,whereas the microporous peak at less than about 2 nm has probablyevolved from exposure of new micropores by carbon dioxide activation.The third peak at about 3 nm may represent preexisting pores or could bedue to activation-induced widening of preexisting micropores. Althoughall samples contain noticeable amount of micropores (diameter <2 nm),carbon cryogels with R/C=25 and 50 possess appreciably greatermicropores volumes than that of samples with R/C=10 and 75. By comparingthe results illustrated in FIGS. 7 and 8, it becomes clear that themicropores play an important role in methane gas storage capacity.Comparison of cryogels having R/C 25 and R/C 50 revealed that the amountof mesopores (2<d<50 nm in diameter) is less important for gravimetricmethane gas storage capacity, but does have an effect on the volumetricstorage capacity. The large mesoporous peak in the R/C 50 cryogel, forexample, is likely responsible for the drop in volumetric capacity ofthat cryogel due to its reduced density.

In one embodiment, carbon cryogels are fabricated from components havingR/C of from about 5 to about 1500. In one embodiment, R/C is from about10 to about 300. In one embodiment, R/C is from about 20 to about 60. Inone embodiment, R/C is from about 25 to about 50.

The effect of R/W on carbon cryogel methane storage capacity isillustrated in FIGS. 9A and 9B. FIGS. 9A and 9B compare the volumetricperformance (V/V) and density (g/cm³) as a function of R/W, and thegravimetric performance (mole/g) and inverse density (cm³/g) as afunction of R/W, respectively, on loose powder samples of representativecarbon cryogels of the invention (Samples 14, 17, 12, 21, and 23, seeTable 1) having R/C 75 and activation between 14% and 36%.

FIG. 9A shows the effect of R/W on nominal powder density and volumetricstorage performance. Increasing the powder density increases the amountof storage material per unit volume, which should increase volumetricperformance uniformly. Theoretically, increasing R/W (amount of organicprecursor per unit volume) should be an effective way of increasing thevolumetric storage capacity. Initially this is the case as V/V andpowder density both increase dramatically when R/W increases from 0.125to 0.25. Then, however, V/V abruptly decreases and then levels off. Thiscan be explained in terms of the relationship between R/W, gravimetricstorage performance, and nominal powder density (FIG. 9B). FIG. 9B showsthat higher densities correspond to lower gravimetric storage capacity,thereby demonstrating that increasing the density of the final activatedcarbon cryogel by increasing R/W is not an efficient way to improve thevolumetric performance. The reduced gravimetric performance implies thatthe additional precursor added for increasing R/W is reducing in termsof the storage efficiency of the final carbon cryogel.

The reduction in gravimetric performance may be explained in terms ofthe rapid gelation that was observed at high R/W. As with very highcatalyst concentrations, at higher R/W, a limit may be reached where asignificant amount of the final carbon cryogel is rendered non-porous.Once a threshold catalyst concentration is reached (for example, R/C75), the high R/W would enable catalyzed crosslink formation to proceedvery rapidly. The close proximity of precursors to one another couldresult in very dense crosslinking. This may be the same effect thatdrove pore sizes in the R/C experiments to the point where porescollapsed and reduced cryogel methane storage capacity.

Referring to FIG. 9A, the leveling of the density curve as R/W increasesmay be explained by a phenomena that was observed during gelation. Infabricating these materials it was noted that for materials at R/C 75and R/W higher than 0.5, the gelation process was very fast (<10 min). Agas was formed during gelation that produced large bubbles in the gel.In a network that forms this rapidly it may be assumed that theformation of large bubbles during gelation might also indicate thepresence of smaller undetectable bubbles that would cause a reduction indensity. It may be possible to counter this effect by reducing thecatalyst to slow down the reaction.

In one embodiment, carbon cryogels are fabricated from components havingR/W of from about 0.01 to about 2.0. In one embodiment, R/W is fromabout 0.1 to about 1.0. In one embodiment, R/W is about 0.25.

The effect of activation on carbon cryogel methane storage capacity isillustrated in FIG. 10. FIG. 10 compares the effect of activation onmethane storage performance (V/V and mole/g) for representative carboncryogels of the invention (Samples 15-19, see Table 1) having R/C 75 andR/W 0.25 cryogels. Referring to FIG. 10, an increase in both gravimetricand volumetric storage capacity with increased activation is initiallyobserved, which is followed by a decline at activation greater thanabout 70%. The improvement in storage capacity on activation can beattributed to increased micropores as activation induces exposure of newmicropores as demonstrated in FIG. 5. The increase in storage capacitycontinues to the point where fewer new micropores are revealed byactivation and, with continued activation, the average pore size beginsto increase and the surface area decreases. As a result, the gas storagecapacity decreases sharply with further increase in activation.

In one embodiment, carbon cryogels are fabricated having activation offrom about 5 to about 90%. In one embodiment, activation is from about25 to about 75%. In one embodiment, activation is from about 60 to about80%.

The present invention provides tunable carbon cryogels that exhibitproperties that are desirable for efficient methane storage. In certainembodiments, the invention provides carbon cryogels having surface areasexceeding 2500 m²/g, a high volume of pores having diameters less thanabout 2 nm, and high methane storage capacities (13.1 mMol/g). Processparameters R/C, R/W, and percent activation can all be used to tune themicroporosity and hence storage performance of carbon cryogels. R/Cplays an important role in determining the microporous structure andhence the methane storage capacity. R/W and activation percentage exerta noticeable influence on both micropore structure and methane storagecapacity. The carbon cryogels of the invention can be used as poroussorbents for low pressure methane storage.

EXAMPLES Example 1 Preparation of Representative Carbon Cryogels

The following chemicals were used in the preparation of carbon cryogels:resorcinol (99+%, Sigma-Aldrich, C₆H₄(OH)₂), formaldehyde solution(37%-stabilized with methanol (CH₃OH), Fisher Scientific, CH₂O), sodiumcarbonate (99.5%, Sigma-Aldrich, Na₂CO₃), trifluoroacetic acid (99%,Aldrich, CF₃CO₂H), and tert-butyl-alcohol (t-butanol) (99.8%, J. T.Baker, (CH₃)₃COH). These were used as received without furthertreatment. A series of carbon cryogels with initial composition listedin Table 1 were fabricated.

The molar ratio of resorcinol to formaldehyde was maintained at 1:2 forall sols, while the molar ratio of resorcinol to sodium carbonatecatalyst (R/C) and the mass or molar ratio of resorcinol to water (R/W)were varied systematically. The sols were prepared by admixingresorcinol and formaldehyde in stirred deionized (DI) water then addingcatalyst at room temperature. The resulting sols were sealed in glassampules or vials and gelled by heating at 90° C. for at least 24 hoursor until gelation was complete (as long as 7 days). No aging was appliedafter gelation. The resulting RF hydrogels underwent solvent exchange toreplace water with t-butanol by rinsing 3 times in fresh t-butanol for24 hours each time followed by subsequent freeze drying for 3 days. Theresulting RF cryogels were pyrolyzed at 1050° C. in nitrogen for 4=hoursand then activated at 900° C. in carbon dioxide with a flow rate of 400SCCM for various durations. FIGS. 1A-1C are images of an RF hydrogel, anRF cryogel, a representative carbon cryogel of the invention,respectively. FIG. 1D is a transmission electron microscope (TEM) image(175,000×) of the carbon cryogel of FIG. 1C.

The initial compositions used to prepare a series of representativecarbon cryogels of the invention are summarized in Table 1.

TABLE 1 Initial molar resorcinol/catalyst ratio (R/C), weightresorcinol/water ratio (R/W by weight), molar resorcinol/water ratio(R/W molar), and level of activation (% Activation expressed as percentoriginal material removed). Sample Number R/C R/W by weight R/W molar %Activation 1 10 0.250 0.041 33 2 10 0.250 0.041 67 3 100 0.250 0.041 304 100 0.250 0.041 42 5 25 0.125 0.020 33 6 25 0.250 0.041 28 7 25 0.2500.041 75 8 300 0.125 0.020 35 9 50 0.050 0.005 12 10 50 0.250 0.041 3511 50 0.250 0.041 68 12 75 0.500 0.082 24 13 75 0.125 0.020 24 14 750.125 0.020 33 15 75 0.250 0.041 7 16 75 0.250 0.041 16 17 75 0.2500.041 36 18 75 0.250 0.041 70 19 75 0.250 0.041 78 20 75 1.000 0.164 021 75 1.000 0.164 14 22 75 1.000 0.164 56 23 75 2.000 0.327 26

Example 2 Preparation of Representative Carbon Cryogels

The representative carbon cryogels prepared as described in Example 1were analyzed by transmission electron microscopy (TEM), and nitrogensorption isotherms.

Methane storage analysis was performed using a Sievert's apparatus shownschematically in FIG. 2. Referring to FIG. 2, apparatus 100 includesbench 10 having rolling track 12, which moves tube furnace 14(Thermolyne Tube Furnace 1200° C. maximum, Bamstead International, 2555Kerper Boulevard, Dubuque, Iowa) in position surrounding chamber 20housing the cyrogel sample to be tested. The chamber includesthermocouple 22. Gas is supplied to and removed from the chamber throughtubing 30 (SS 316L AMS269, seamless, ¼ inch, Swagelok, 29500 Solon Road,Solon, Ohio) and regulated by valves 40 (Swagelok SS4BK). Pressure wasmeasured by high pressure transducer 50 (Setra 225 1000 psi maximum,Setra, 159 Swanson Road, Boxborough, Mass.) and by low pressuretransducer 60 (MKS D27B13TCECOBO, 45° C. dual range having 1000/100 torraccuracy of 0.12% reading, MKS Instruments, 90 Industrial Way,Wilmington, Mass.). Gas entering the chamber was passed through filter70 (0.5 micron stainless steel filter). Gas was delivered to the chamberfrom gas tank 80. The chamber can be vented to fumehood 92. The chambercan be evacuated by vacuum pump 94 through trap (e.g., liquid nitrogen)96.

Methane storage analysis using a Sievert's apparatus shown schematicallyin FIG. 2 was performed as follows. The samples were loaded into a glassslip inside a stainless steel tube without compaction and evacuated at atemperature of 200° C. prior to an adsorption test with high puritymethane (99.99%). The amount of methane adsorbed was measured by dosingwith known volumes of methane at stepwise increasing pressures up toabout 4.5 MPa. Adsorption curves expressed in terms of volumetricperformance (V/V) (volume of stored gas at standard temperature andpressure (STP) divided by volume of sample) and gravimetric performance(mole/g) (moles of methane adsorbed per gram of carbon) were calculated.

Adsorption storage capacities determined for representative carboncryogels of the invention (Samples 1-23, Example 1) are summarized inTable 2.

TABLE 2 Activated carbon cryogel volumetric and gravimetric methanestorage capacity and powder density. Sample Number V/V mol/g density %gravimetric 1 60 0.0039 0.62 6.28 2 53 0.0029 0.75 4.63 3 85 0.0089 0.3914.30 4 76 0.0069 0.45 11.00 5 70 0.0123 0.23 19.80 6 100 0.0064 0.6310.28 7 111 0.0119 0.38 19.08 8 22 0.0010 0.92 1.63 9 63 0.0127 0.2020.36 10 91 0.0066 0.56 10.63 11 94 0.0131 0.29 21.05 12 83 0.0041 0.826.62 13 73 0.0104 0.29 16.69 14 69 0.0096 0.30 15.36 15 92 0.0050 0.748.07 16 104 0.0056 0.76 8.93 17 103 0.0067 0.63 10.75 18 118 0.0063 0.7510.18 19 101 0.0056 0.73 9.03 20 78 0.0040 0.80 6.37 21 93 0.0055 0.698.80 22 96 0.0055 0.71 8.80 23 90 0.0049 0.74 7.90

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A carbon material comprising a surface area ranging from 1500 m²/g to3000 m²/g, wherein the carbon material comprises a pore sizedistribution comprising: a) pores having a diameter less than 2 nm; b)pores having a diameter of 3 nm; and c) pores having a diameter between7 and 8 nm.
 2. A carbon material comprising a gravimetric methanestorage capacity ranging from 15% to 20%, a surface area ranging from2400 m²/g to 2600 m²/g and a total pore volume ranging from 1.0 cm³/g to1.5 cm³/g.
 3. A carbon material comprising a peak pore volume greaterthan 0.1 cm³/g for pores comprising a diameter less than 2 nm andcomprising a peak pore volume greater than 0.1 cm³/g for porescomprising a diameter ranging from 5 nm to 12 nm.
 4. The carbon materialof claim 3 further comprising a peak pore volume greater than 0.04 cm³/gfor pores comprising a diameter ranging from 2 nm to 4 nm.
 5. A carbonmaterial comprising a gravimetric methane storage capacity ranging from0.009 mol/g to 0.013 mol/g at room temperature and 500 psi, a surfacearea ranging from 1500 m²/g to 2500 m²/g and a total pore volume rangingfrom 1.2 cm³/g to 1.5 cm³/g.
 6. The carbon material of claim 5, whereinthe total pore volume ranges from 1.4 cm³/g to 1.5 cm³/g.
 7. The carbonmaterial of claim 5, wherein the pore volume ranges from 1.2 cm³/g to1.4 cm³/g.
 8. The carbon material of claim 5 comprising mesoporescomprising diameters ranging from 5 nm to 12 nm.
 9. The carbon materialof claim 5 comprising mesopores comprising diameters ranging from 2 nmto 4 nm.
 10. The carbon material of claim 5, wherein the surface arearanges from 1500 m²/g to 2000 m²/g.
 11. The carbon material of claim 5,wherein the surface area ranges from 2000 m²/g to 2500 m² 2/g.
 12. Amethod for making the carbon material of claim 5, the method comprising:(a) preparing a sol by mixing resorcinol, formaldehyde, and a catalystin water; (b) gelling the sol by heating at a temperature and for a timesufficient to provide a gel; (c) washing the gel with acid to provide anacid-washed gel comprising an aqueous solvent; (d) washing theacid-washed gel with a suitable organic solvent to exchange the aqueoussolvent for an organic solvent to provide a solvent-exchanged gel; (e)freeze drying the solvent-exchanged gel; and (f) pyrolyzing thesolvent-exchanged gel to provide a pyrolyzed carbon material.
 13. Themethod of claim 12, wherein the molar ratio of resorcinol to catalystranges from 10 to
 300. 14. The method of claim 12, wherein the catalystis sodium carbonate.
 15. The method of claim 12, wherein the weightratio of resorcinol to water ranges from 0.01 to 2.0.
 16. The method ofclaim 12, wherein gelling the sol comprises heating at a temperature andfor a period of time sufficient to convert the sol to a crosslinked gel.17. The method of claim 12, wherein gelling the sol comprises heating at90° C. for a time ranging from 1 to 7 days.
 18. The method of claim 12,wherein washing the gel with acid comprises washing the gel with aqueoustrifluoroacetic acid.
 19. The method of claim 12, wherein washing theacid-washed gel comprises washing the gel with t-butanol.
 20. The methodof claim 12, wherein pyrolyzing the solvent-exchanged gel comprisesheating at a temperature and for a period of time sufficient to convertthe gel to a pyrolyzed carbon material.
 21. The method of claim 12,wherein pyrolyzing the solvent-exchanged gel comprises heating at 1050°C. for 4 hours under nitrogen.
 22. The method of claim 12, furthercomprising heating the carbon material at a temperature and for a timesufficient to provide an activated carbon material.
 23. The method ofclaim 22, wherein the temperature is 900° C., and the time ranges from10 to 120 minutes.
 24. The method of claim 22, wherein the carbonmaterial is activated from 5% to 90%.
 25. The method of claim 22,wherein heating at a temperature and for a time sufficient to provide anactivated carbon material comprises heating at 900° C. under carbondioxide.